SWR did that Amp-Lite and Head-Lite thing, where it was a 1U half rack space power amp that you could add on to your 1U half rack amp to double the power.
There are lots of threads on here about 1U power amps, but general consensus seem to be that there is just not enough market interest to actually produce one. There are a few out there, but they tend to be companies you've never heard of, or super expensive, or super deep (like the stewart world 1.2).
Demeter makes one. Jule amps makes one and just released (or is about to) a 500w "pedal board" amp. But those are both pretty $$. Lots of dudes using the Quilter BB800 as a power amp - had a direct in on the back for that.